The Earth Science Picture of the Day (EPOD) highlights the diverse processes and phenomena which shape our planet and our lives. EPOD will collect and archive photos, imagery, graphics, and artwork with short explanatory captions and links exemplifying features within the Earth system. The community is invited to contribute digital imagery, short captions and relevant links.

Lidar Profile of a Passing Cold Front

Looking up into the sky with lasers, scientists captured this view of a passing cold front during an experiment near Lamont, Oklahoma in 1994. Using a technique called LIDAR (Light Detection and Ranging), pulses of laser light are beamed into the sky. Some of the laser light scattered by the atmosphere returns to sensitive detectors on the ground, and the time it takes to return is measured to determine the altitude of the scattering particles. The NASA Goddard Scanning Raman Lidar (SRL) used in the Lamont experiment also measures the small shift in wavelength of the laser light as it interacts with vibrating water molecules (Raman scattering), enabling water molecule concentrations to be measured.

The colors in the image represent the water vapor mixing ratio (grams of water per kilogram of air) of the atmosphere as measured by the SRL. Individual measurements over a period of nine hours were combined to create the above profile of the cold front. As the front passed over, the vertical distribution of water vapor changed. Very moist air (reds and greens) precedes the front, and rides up the tongue of colder drier air (light blue) as the front passes. Clouds are represented by "bricks" and the cold frontal surface and the moisture front are identified by the magenta colored boundary line. Behind the cold front (on the right) is an atmospheric phenomenon characterized by waves traveling in a fixed pattern behind a wave-front called an undular bore.